Commutator-brush.



No. 792,739. PATENTED JUNE 20, 1905. G. W. SPEIRS.

GOMMUTATOR BRUSH.

APPLICATION FILED NOV. 7, 1904.

Inventor CA 073W Patented June 20, 1905.

FFICFa CHARLES WILLIAM SPEIRS, OF BATTERSEA, ENGLAND, ASSIGNOR TO MORGAN ORUOIBLE COMPANY, LIMITED, OF BATTERSEA, LONDON,

ENGLAND.

COMMUTATOR-BRUSH.

SPECIFICATION forming part of Letters Patent No. (132/739, dated June 20, 1905.

Application filed November '7, 1904. Serial No. 231,820.

To all who/n it may concern:

Be it known that I, OnARLns WVILLIAM Srnrns, a subject of the King of Great Britain, residing at Battersea Works, Battersea, in the county of London,England, have invented new and useful Improvements in Oommutator- Brushes, of which the following is a specification.

This invention relates to the manufacture of carbon commutator-brushes for electrical generators or motors.

It is well known that the high electrical conductivity of plumbago renders it especially suitable for commutator-brushes; but hitherto there have been considerable practical difiiculties in producing brushes of this material for commercial purposes. The chief qualities of a good brush are a minimum of resistance in the longitudinal direction or direction of the flow of the current and a high resistance at right angles to the direction of flow. Hitherto in manufacturing plumbago brushes it has been usual to mix the plumbago with various substances to form a plastic mass, which is subsequently molded and fired; but the substance added, and WlllClIIS usually of the nature of clay, has the effect of increasing the internal resistance of the brush.

I have found as the result of numerous experiments that by grinding pure crystalline or flaked natural plumbago (such as may be obtained from Ceylon) to a suitable fineness for instance, to pass through a sieve having one hundred meshes to the inch and subjecting the same to pressure in a dry state I can dispense with the formation of a plastic mass and the subsequent firing and at the same time obtain solid blocks having a high conductivity in a direction at right angles to the applied pressure and a relatively low anduniform conductivity in the direction of the pressure.

In order to impart sufficient hardness to the brushes and retain the qualities of conductivity, as above stated, I treat the powdered plumbago with a glutinous substance, such as gelatin in solution, and then drive off the moisture, leaving each particle coated with good results.

the gelatin, the mass being again subjected to a grinding operation to insure that the particles do not adhere. In practice I have found. that a solution made by dissolving one pound of gelatin in one hundred pounds of water and mixed with the ground plumbago in the proportion of one and one-half parts of the former to one part of the latter, by weight, gives I/Vhen now the dry mass is sub jected to the requisite pressure-say, for instance, twenty tons to the square inchthe air between the particles is driven out laterally and causes the said particles (which, notwithstanding the grinding, are still in the form of flakes) to arrange themselves with their larger surfaces parallel with the pressing-surface.

The longitudinal conductivity of a brush constructed as hereinbefore described is slightly lower than that of pure flake plumbago; but the transverse conductivity is considerably lower, and I believe this to be due to the fact that under the heavy pressure which I employ the particles are expanded in the direction of their planes and burst the gelatin, making good electrical contact with one another at their edges, while the gelatin between the planes of the various particles increases the resistance in that direction. A

To illustrate my invention, I append a sheet of drawings, in which Figure 1 is a front elevation, Fig. 2 a side elevation, and Fig. 3 a plan View, of a commutator-brush.

In the figures, a (4 indicate the flakes of plumbago, which, for the sake of clearance are drawn to a greatly-exaggerated scale. In making the brush the ground plumbago is introduced into a mold and then subjected to pressure in the directionof the arrow, Fig. 3, so that under the pressure the flakes will, in the manner hereinbefore indicated, arrange themselves with their planes or flat surfaces parallel with the pressing-surface, the largerdimensions of the flakes being in the direction of the length of the brush, as indicated in Figs. 1 and 2. hen the flakes are arranged in this manner, it will be understood that a much greater number of flakes will lie face to face in the thickness of the brush than will lie edge to edge in a correspondinglength in the longitudinal direction of the brush, and as the surfaces in contact ofler a certain amount of resistance to the flow of current it will be obvious (the number of contact-surfaces being greater in the thickness than in the length of the brush) that there will be much greater resistance in the former than in the latter direction, so that the current will flow much more readily in the direction of the arrows 1, Fig. 2, than of the arrows 2 in the said Fig. 2. Results obtained in actual practice show i that the specific resistance is from ten to forty times greater across the grain than it is along the grain.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that What I claim is A commutator-brush formed of plumbago of a nature which when ground is in the condition of small flakes, the ground plumbago being treated with a glutinous substance and subjected to great pressure in a mold in a direction at right angles to that in which the current is to flow through the brush, whereby the flakes will arrange themselves with their planes parallel with the pressing-surface so that a relatively large number of contacts between the flakes will occur in the thickness of the brush compared with the number of contacts between the flakes in the length of the brush, substantially as described.

CHARLES WILLIAM SPEIRS.

Witnesses:

C. G. REDFERN, A. ALBUTT. 

